The Phonic Wheel was invented by L.R. Wilberforce (best known for the Wilberforce pendulum, 1894) for the purpose of determining the frequency of tuning forks. Although I have not seen the instrument in operation, it appears to be a stroboscope. A beam of light from an external source is reflected from the mirror, and passes through one of the thirty slots in the rotating wheel. The speed of the rotating wheel can be controlled, and measures are taken to ensure that the rotation speed is constant. A bell rings every ten revolutions of the wheel.

   The tuning fork is illuminated by the light from the instrument, and the rotation rate is adjusted until the tines of the tuning fork appear to be at rest. The frequency of the fork can then be obtained by using a stopwatch to find the time between rings of the bell.

   The instrument is signed "Lucas"; Mollan notes that a similar apparatus at Trinity College Dublin is signed "Veeder M'f'g Co. Hartford, Conn USA" and was patented in 1899. It is listed at £5 in the 1911 catalogue of W. G. Pye & Co. of Cambridge, England.